The development of new portable electronic products, such as the laptop computer, is currently of great worldwide interest. Miniaturization of the various component systems (memories, displays, and so forth) for such products requires that the necessary circuits be packed in as small a volume as possible. Packing circuits into a small volume also reduces parasitic capacitance and improves signal propagation time between circuits. One approach to this requirement is to increase the scale of integration in order to obtain all of the required functions from a circuit made from a single wafer. Unfortunately, efforts to create full-wafer circuitry have encountered unacceptable yield losses owing to the large circuit size. In the specific area of active matrix displays, a similar problem results in attempting the scale-up of the display size to and beyond the 256K pixel level.
Active matrix (AM) displays generally consist of flat-panels consisting of liquid crystals or electroluminescent materials which are switched "on" and "off" by electric fields emanating from pixel electrodes charged by thin-film transistors (TFT's) co-located with each liquid crystal or electroluminescent pixel area. These AM displays are expected to supplant cathode ray tube (CRT) technology and provide a more highly defined television picture or data display. The primary advantage of the active matrix approach, using TFT's, is the elimination of cross-talk between pixels, and the excellent grey scale that can be attained with TFT-compatible liquid crystal displays (LCD's).
Flat panel displays employing LCD's generally include five different layers: a white light source layer, a first polarizing filter layer that is mounted on one side of a circuit panel on which the TFT's are arrayed to form pixels, a filter plate layer containing at least three primary colors arranged into pixels, and finally a second polarizing filter layer. A volume between the circuit panel and the filter plate is filled with a liquid crystal material. This material rotates the polarization of light passing through it when an appropriate electric field is applied across it. Thus, when a particular pixel electrode of the display is charged up by an associated TFT, the liquid crystal material rotates polarized light being transmitted through the material so that it will pass through the second polarizing filter and be seen by the viewer.
The primary approach to TFT formation over the large areas required for flat panel displays has involved the use of films of amorphous silicon which has previously been developed for large-area photovoltaic devices. Although the TFT approach has proven to be feasible, the use of amorphous silicon compromises certain aspects of the panel performance. For example, amorphous silicon TFT's lack the frequency response needed for large area displays due to the low electron mobility inherent in amorphous material. Thus, the use of amorphous silicon limits display speed, and is also unsuitable for the fast logic needed to drive the display.
Owing to the limitations of amorphous silicon, other alternative materials are being considered, such as, polycrystalline silicon, or laser recrystallized silicon. Thin films, less than about 0.4 microns, of these materials are usually formed on glass which generally restricts further circuit processing to low temperatures.
The formation of large active-matrix displays is hampered by the unavailability of large-area single crystal Si material. Thus the conventional approach is to use thin-film amorphous (.alpha.-Si) or polycrystalline Si (poly-Si) wafers. The required number of thin-film transistors (TFT's), combined with the large number of driver circuits and the thin-film material defects inherent in .alpha.-Si or poly-Si, leads to unacceptable yield and quality problems when the entire display is to be fabricated as a unit.
A need exists, therefore, for a relatively inexpensive way to reliably form hybrid high density electronic circuits, including active matrices, memories, and other devices, in a modular approach that permits small high-quality parts or circuits to be assembled into complete large-area high-quality complex devices.